The present invention relates to the processing of semiconductor wafers. More particularly, the present invention relates to a borophosphosilicate glass layer formed over a semiconductor substrate and an improved method and apparatus for forming the same. The present invention is particularly useful in the formation of a premetal dielectric layer but may also be applied to the formation of intermetal dielectric layers, passivation layers, and the like.
Silicon oxide is widely used as an insulating layer in the manufacture of semiconductor devices. A silicon oxide film can be deposited from a reaction of silane (SiH4), tetraethylorsilicate (Si(OC2H5)4), hereinafter referred to as xe2x80x9cTEOS,xe2x80x9d or a similar silicon containing source with an oxygen containing source such as nitrous oxide (N2O), molecular oxygen (O2), ozone (O3), or the like.
One particular use for a silicon oxide film is as a separation layer between the polysilicon gate/interconnect layer and the first metal layer of MOS transistors. Such separation layers are referred to as premetal dielectric (PMD) layers because they are typically deposited before any of the metal layers in a multilevel metal structure. In addition to having a low dielectric constant, low stress and good adhesion properties, it is important for PMD layers to have good stability, moisture resistance, and planarization characteristics.
When used as a PMD layer, the silicon oxide film is deposited over a lower level polysilicon gate/interconnect layer that usually contains raised or stepped surfaces. The initially deposited film generally conforms to the topography of the poly layer and is typically planarized or flattened before an overlying metal layer is deposited. A standard reflow process, in which the oxide film is heated to a temperature at which it flows, may be employed to planarize the film. Alternatively, a chemical mechanical polishing (CMP) or etching technique may be used.
Because of its low dielectric constant, low stress, good adhesion properties and relatively low reflow temperature, borophosphosilicate glass (BPSG) is one silicon oxide film that has found particular applicability in PMD layers. One known method for forming a BPSG layer uses a plasma enhanced chemical vapor deposition (PECVD) process in which phosphorus and boron containing sources are introduced into a processing chamber along with the silicon and oxygen sources normally required to form a silicon oxide layer. In this process, high frequency RF energy (13.56 MHz) is applied to a reaction zone proximate the substrate surface to promote excitation and/or disassociation of the reactant gases and thereby create a plasma of highly-reactive species. The high reactivity of the released species reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such PECVD processes.
Semiconductor integrated circuits currently being manufactured follow ultra high density (e.g., about 0.5 to 0.35 micron) design rules and circuits manufactured in the near future will follow even smaller design rules. At such small feature sizes, issues that are important in the formation of BPSG layers include a film""s ability to resist moisture absorbtion, film stability and film reflow temperature among others. The temperature at which PMD layers are reflowed is an important aspect of a process thermal budget. Temperatures above a certain level during this step can destroy shallow junctions, degrade self-aligned titanium silicide contact structures and create other problems.
One method of lowering the reflow temperature of BPSG films is to incorporate more dopant, particularly boron, into the film. Incorporating higher levels of dopants in the BPSG film using known PECVD such as the single frequency process described above, however, adversely effects the stability and the moisture resistance properties of some BPSG films. Thus, it is desirable to produce BPSG films that have a higher dopant concentration level (and thereby a reduced reflow temperature) that also are stable and have good moisture resistant properties.
Other PECVD processes apply mixed frequency RF power to generate a plasma where separate high and low frequency power supplies are used. These processes are known to be used in the deposition of undoped silicate glass (USG) layers, fluorine-doped silicate glass (FSG) layers and other films. Mixed frequency PECVD processes, however, have not been used in the deposition of BPSG films.
The present invention provides a borophosphosilicate glass (BPSG) film, and method and apparatus for forming the same, that has improved stability and moisture resistance at high dopant concentration levels and that has improved reflow properties. The BPSG film according to the present invention is formed under plasma conditions in which high and low frequency RF power is employed to generate the plasma. A high frequency power supply provides most of the energy to form the plasma and promote the necessary reactions. A low frequency power supply regulates and controls ion bombardment of the BPSG film as it is formed.
The method of the present invention includes the steps of introducing a process gas including boron, phosphorus, silicon and oxygen into a processing chamber and applying mixed radio frequency energy to the chamber to form a plasma from the process gas and deposit a BPSG film. The mixed frequency RF energy has separate high and low frequency components.
In a preferred embodiment of the method of the present invention, nitrogen is added to the process gas and the mixed frequency RF energy is supplied from separate high and low frequency power supplies. The low frequency RF power supply is used to control ion bombardment during deposition processing. Precise control of ion bombardment allows incorporation of an unexpectedly elevated amount of nitrogen into the film, which further improves film stability. In a still more preferred embodiment, the process gas includes nitrous oxide as a source of both oxygen and nitrogen.
These and other embodiments of the present invention along with many of its advantages and features are described in more detail in conjunction with the text below and attached figures.